Developmentally and physiologically controlled cell death can be observed in almost all tissues of various species of animals. Such cell deaths are generally considered `programmed` and are distinguishable from `accidental` deaths that occur by pathological mechanisms. Most of the cells undergoing programmed death have been shown to require de novo synthesis of RNA and protein.
These facts suggest that at least a few genes, if not specified ones, must be expressed to cause programmed cell death.
The term "apoptosis", on the other hand, is used to describe the morphological characteristics of a class of cell death. In cells dying by apoptosis, chromatin condenses around the periphery of the nucleus, while mitochondria and other organelles are unaffected. A unique biochemical feature of apoptotic cells includes fragmentation of DNA into oligonucleosomal pieces. In mammals, apoptosis is often morphologically and biochemically associated with programmed cell death, but some of the cells undergoing programmed death apparently do not show the characteristic features of apoptosis. In addition, there are apoptotic cell deaths that can be induced in the absence of any protein synthesis.
Thus, it is important to note that apoptosis is not synonymous with programmed cell death.
It has recently become apparent that bcl-2, which is a oncogene, protects mortalized B cells from cell death, thus showing its importance to control cell death.
Certain peptides that are related to programmed cell death have been reported, such as the Fas antigen (Itoh et al, Cell, 66:233 (1991)).
Human Fas antigen is a polypeptide consisting of about 335 amino acids, having a signal peptide consisting of 16 hydrophobic amino acids N-terminal. Its mature protein is thought to have a structure divided into an extracellular domain (157 amino acids), a transmembrane region (17 amino acids) and a cytoplasmic domain (145 amino acids). It is thought to function as a receptor to a factor (ligand) inducing cell death.